The present invention relates generally to an organic EL (electroluminescent) device, and specifically to an improvement in or relating to a hole injecting electrode for feeding holes (charges) to a light emitting layer.
In recent years, organic EL devices have been under intensive investigation. One such device is basically built up of a thin film form of hole transporting material such as triphenyldiamine (TPD) deposited by evaporation on a hole injecting electrode, a light emitting layer of fluorescent material such as an aluminum quinolinol complex (Alq.sup.3) laminated thereon, and a metal (electron injecting) electrode of a metal having a low work function such as Mg and formed on the light emitting layer. This organic EL device now attracts attentions because a very high luminance ranging from several hundreds to tens of thousands cd/m.sup.2 can be achieved with a voltage of approximately 10 V.
A hole injecting electrode considered to be effective for such organic EL devices is made up of a material capable of injecting more electrons into a light emitting layer or a hole injecting and transporting layer. One requirement for the material is that it be transparent and electrically conductive because, in most cases, an organic EL device is usually designed to take out the emitted light from a substrate side thereof.
ITO (tin-doped indium oxide), IZO (zinc-doped indium oxide), ZnO, SnO.sub.2, In.sub.2 O.sub.3, etc. have been known for such transparent electrodes. Among these, ITO electrodes have a visible light transmittance of 90% or higher and a sheet resistance of 10 .OMEGA./.quadrature. or lower, and so find wide applications as transparent electrodes for liquid crystal displays (LCDs), dimmer glasses, and solar batteries. The ITO electrodes are also considered to be promising for hole injecting electrodes in the organic EL devices.
An organic EL device requires a given current for light emission, and the light emission luminance increases in proportion to an applied current density. Consequently, when sophisticated display patterns or large-screen displays are driven with high luminance or at a high duty ratio, the resistance of interconnecting lines in the hole injecting electrode is found to give rise to a voltage drop problem, although this resistance is negligible when the length of interconnecting lines is short. If, for example, a display of 256 dots.times.64 dots is driven at a light emission luminance of 150 cd/m.sup.2, a 1/64second-light emission will occur at a light emission luminance of 150.times.64 cd/m.sup.2. When the sheet resistance of the hole injecting electrode is sufficiently reduced, the device may be driven at a voltage value approximate to an effective applied voltage shown in FIG. 15 for instance. When the resistance of the hole injecting electrode is about 260 .OMEGA., however, a voltage increase of as high as 2 V is needed, as can be seen from the applied voltage shown in FIG. 15. Even with an electrode having a sheet resistance of the order of 7 to 8 .OMEGA./.quadrature. such as a low resistance ITO electrode, a voltage drop across the hole injecting electrode becomes a grave problem because a resistance of 64.times.7=448 .OMEGA. exists even in a narrow pixel area of 64 dots.